Ivanka Trump used her personal email account to send hundreds of government emails last year, according to US media reports.
The daughter of the US president and top White House adviser sent emails to White House aides, cabinet members and assistants – many in violation of public records rules, the Washington Post said, citing people familiar with a White House examination of her correspondence.
The discovery was made when White House ethics officials were reviewing emails gathered last year by five cabinet agencies to respond to a public records lawsuit.
The review revealed that throughout 2017, Ms Trump often discussed or relayed official White House business using a private email account with a domain that she shares with her husband, Jared Kushner.
During the 2016 presidential campaign, Donald Trump harshly criticised his Democratic opponent Hillary Clinton for using her personal email account for government business when she was secretary of state.
Mr Trump labelled her "crooked Hillary" and said she belonged in jail.
Ms Clinton blamed her defeat on then-FBI director James Comey re-opening an investigation into her emails 11 days before the election.
She was eventually cleared of any crime.
In late February 2017, Ms Trump used her personal email to contact Small Business Administration chief Linda McMahon and propose they meet to explore "opportunities to collaborate."
The following month, she emailed Education Secretary Betsy DeVos, suggesting that their staffers meet to discuss ways to collaborate on "locational/workforce development and k-12 STEM education."
While her messages were largely about government work, Ms Trump was not then subject to White House records rules.
A spokesman for Ivanka Trump's lawyer said: "While transitioning into government… Ms Trump sometimes used her private account, almost always for logistics and scheduling concerning her family."
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He stressed that no classified information was transmitted in the messages, that no emails were deleted and that the emails have since been "retained" in conformity with records laws.
"When concerns were raised in the press 14 months ago, Ms Trump reviewed and verified her email use with White House Counsel and explained the issue to congressional leaders," he said.